The present invention is directed to web-tensioning brakes for roll stands and in particular to brakes of the type that act essentially as generators connected to loads so that, when they are driven by the rotating roll, they exert a drag on it and thereby apply tension to web material being unwound from the roll.
Many industrial processes that convert sheet material to finished goods start with a roll of material supported on a roll stand, from which the sheet material is unwound. For most such processes, one of the process variables whose control is important is the web tension. For this purpose, brakes on the unwind roll stand resist its rotation and thereby tension the web.
Most brakes used for this purpose are friction brakes actuated by pneumatic, hydraulic, or electromagnetic means. Brake pads and rotor disks or drums wear when friction brakes are employed, and they require frequent attention. They also produce dust that can contaminate the workplace and the product. And some installations require forced-air or water cooling to keep the brake temperature at a safe level and reduce the rate at which the brakes wear.
In comparison with friction brakes, then, generator-type brakes would appear to have significant advantages. Clearly, generator-type brakes produce little wear and dust in comparison with friction brakes. Furthermore, the power extracted by the generator, being in the form of electricity, may be readily conveyed to a remote ballast resistor for safe dissipation to ambient air or applied to other process uses. A further advantage of generator-type brakes relative to pneumatic or hydraulic types is that their torque can be rapidly varied by direct electronic means, so the tension-control system is potentially more responsive than it would be if it employed electro-pneumatic or electro-hydraulic pressure modulators and brake-pad actuators, which depend to some extent on the movement of mechanical parts such as brake calipers. So it is not surprising that numerous proposals have been made over the years to employ generator-type brakes for this purpose.
Despite a number of such proposals, however, the friction brake has been the predominant, although not exclusive, type used for web tension control of all but the largest roll sizes. Even for large systems employing generator-type brakes, the cost savings in comparison with friction brakes have in some cases been disappointing.
Regardless of whether the brake is of the friction or the generator type, the control system must operate it in such a manner as to keep tension at a desired level despite, for instance, changes in roll radius as the web material is paid out. An early example of such a control system is that described in U.S. Pat. No. 2,052,788 to Miller, which was based on the recognition that keeping brake power constant in a constant-web-speed process will result in constant web tension. Since power is the product of torque and angular velocity, Miller placed an inductor in the generator load circuit so that generator output current--and thus generator torque--would decrease as the roll's angular speed--and thus the generator output frequency-increased with the reduction in roll radius that occurs as the roll pays out the web.
An analogous approach for friction brakes is practiced in control systems that employ a sensing arm or ultrasonic sensor to observe roll radius and decrease brake torque as the radius thus observed decreases so as to avoid the tension increase that would otherwise result.
The Miller and roll-radius-sensing arrangements are both open-loop brake-torque control systems. Being used when particularly high tension accuracy is not required, they are based on measurements, like generator current and roll radius, that are relatively inexpensive to make. When tension-control is required, however, designers have turned to systems that control tension more directly.
One such approach employs a dancer roll, i.e., a roll that can move up and down or fore and aft and that is so loaded, whether by gravity or, for instance, by pneumatic cylinders, that it applies a constant tensioning force to the web so long as the roll is maintained at the central point of its operating range. The brake-torque-control strategy for dancer-roll systems is to sense the dancer-roll position and so control the brake torque as to keep the dancer-roll position substantially constant.
Although dancer-roll systems can be relatively accurate when faced only with fairly slow system variations, they ordinarily are afflicted with certain system lags that make them less responsive to wide-band disturbances such as those that result from roll eccentricity. Moreover, the need to install the dancer roll can make the use of such a system impractical for retrofit purposes because there may be no room for the extra equipment. Even when installed as original equipment, such an approach can be quite expensive. Dancer rolls and supporting bearings and shaft hangers are costly. This is particularly true for wide webs, for which the dancer rolls must be of substantial diameter in order to avoid unreasonable deflections. The expense problem can be multiplied in circumstances in which, in order to obtain the necessary accuracy, the requisite wrap angle can be insured only by providing further, idler rollers.
To eliminate dancer roll's lag problems, which largely stem from roll inertia, some installations make direct tension measurements by employing an idler roller and a load cell that measures radial loads on the idler roller's bearings that result from web tension. The resultant output is compared with a target value, and brake force applied is based on the error output of the comparison. Such systems have at least been advertised to yield high accuracies, and they respond faster than dancer-roll arrangements. But they are subject to much the same retrofit difficulties as dancer-roll systems, and they are usually at least as expensive.